This invention relates to the thin copper foils which are used in manufacturing printed circuit boards (PCB). More particularly, the invention relates to copper foils having improved ability to resist the abrasion which leads to the formation of deep scratches and tears during handling.
It has been found, particularly in more demanding applications where relatively thin copper foils are used (e.g. one-half ounce/ft.sup.2 or 18 .mu.m thick), that when copper foil surfaces are exposed, that they can be damaged by abrasion, especially those which are relatively soft, such as copper foils which exhibit high percentage elongation at elevated temperatures. When scratches become severe, they can lead to tears in the foil and thus can cause rejection of the laminate or the partially completed PCB. In some cases, laminates may be rejected for their appearance even though severe damage has not occurred. Consequently, it is desirable to avoid abrasion damage insofar as possible.
Another common problem in the manufacture of PCBs is "resin spots". These are particles of resins, such as epoxy resins, adhering to the surface of the foil. Such particles interfere with the etching process used to make circuit lines and can lead to defects which cause rejection or reworking of a circuit board. Coating copper foils with thin layers of benzotriazole (BTA) has been found to reduce the number of "resin spots", but further improvement is needed in the industry, particularly when fine circuit lines are being made and very small resin particles are present.
It has been the practice in the industry to treat copper foils to protect them from corrosion during the period between their manufacture and their use in making printed circuit boards. One such treatment has been to apply benzotriazole and/or related compounds. For example, in U.S. patent application Ser. No. 08/496,502 now abandon both benzotriazole and aminotriazole or their derivatives were applied to rust-proof copper foil. Under the rust-proofing layer in the typical case would be a layer of electrodeposited zinc or a zinc alloy, followed by a chromate layer. It also was found that powdered epoxy resin did not adhere to the foil. The use of benzotriazole or aminotriazole alone was not as satisfactory
U.S. patent application Ser. No. 08/686,574 now abandoned discloses the use of aminotriazole and/or its isomers to resist adherence of epoxy resin particles and to resist discoloration when soldering.
In Japanese patent application 1994-85417 the use of benzotriazole or its derivatives was shown to improve solderability and adhesion of resists on the shiny side of the foil.
In Japanese patent application 1994-85455 the use of benzotriazole or its derivatives was shown to improve resistance to moisture and thus the storage life of a treated copper foil. As with the 85417 application, the treatment was placed on the shiny side of the foil. The foil was first plated with zinc or a zinc alloy, then a chromate treatment was added, followed by coating with the benzotriazole.
The present inventors have been concerned with preventing damage to copper foils during handling and have found a method which substantially improves their abrasion resistance. The copper foils made according to the invention have the added advantage of further reducing the number of resin particles adhering to the exposed copper foil.